Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

Disclosed is a method for performing gas-shielded pulsed arc welding at
high current densities with a flux-cored wire as an electrode wire. The
pulsed arc welding is carried out by passing a pulsed current so that a
pulse peak current density during a pulse peak time Tp is 400 to 950
A/mm2, a pulse base current density during a pulse base time Tb is
200 A/mm2 or more and differs from the pulse peak current density by
200 to 400 A/mm2, and an average current density is 350 to 750
A/mm2. The method allows significant spatter reduction while
attaining a high deposition rate.

Claims:

1. A method for high-current-density gas-shielded arc welding, the method
comprising the step of: performing pulsed arc welding with a flux-cored
wire as an electrode wire, wherein the pulsed arc welding is carried out
by passing a pulsed current so that a pulse peak current density during a
pulse peak time Tp is 400 to 950 A/mm2, a pulse base current density
during a pulse base time Tb is 200 A/mm2 or more and differs from
the pulse peak current density by 200 to 400 A/mm2, and an average
current density is 350 to 750 A/mm.sup.2.

2. The method according to claim 1, wherein the shielding gas is a
gaseous mixture containing carbon dioxide (CO2) in a content of 5 to
35 percent by volume with the remainder being argon (Ar).

3. The method according to claim 1, wherein the flux-cored wire comprises
a steel sheath; and a flux filled in the sheath, and wherein the
flux-cored wire has a flux filling rate of 10 to 25 percent by mass based
on the total mass of the wire and contains carbon (C) in a content of
0.08 percent by mass or less, silicon (Si) in a content of 0.5 to 1.5
percent by mass, manganese (Mn) in a content of 1.5 to 2.5 percent by
mass, and titanium (Ti) in a content of 0.1 to 0.3 percent by mass.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a method for high-current-density
gas-shielded arc welding, which is used typically in single-layer or
multilayer welding within a fillet or groove typically in the field of
transportation equipment and construction equipment.

BACKGROUND ART

[0002] In the field typically of transportation equipment and construction
equipment, single-layer or multilayer welding in a fillet or groove is
often performed through gas metal arc welding. This technique employs a
high welding current to perform welding at a higher wire melting speed so
as to achieve a higher efficiency. However, such an increased current
density causes generation of spatters in large amounts, and this causes
inferior appearance of the weld beads and requires an extra step for
removing the spatters, resulting in an insufficient production
efficiency.

[0003] Solid wires are generally used in the field. However, when used in
welding at a high current density of 300 A/mm2 or more, the solid
wires present a droplet transfer mode called rotating transfer.
Specifically with reference to FIG. 2A, excessive Joule heating in a wire
extending portion 1 ranging from a contact tip to an arc generation spot
causes the wire to be softened and melted to thereby elongate from its
tip, and the resulting tip molten metal 2 transfers while rotating with
an arc 4.

[0004] In addition to the rotating transfer, exemplary droplet transfer
modes further include globular transfer as illustrated in FIG. 2B in
which a molten droplet 3 having a size larger than the outer diameter of
the wire extending portion 1 transfers while being repelled; and spray
transfer as illustrated in FIG. 2C in which a molten droplet 3 having a
size smaller than the outer diameter of the wire extending portion 1
transfers. In the rotating transfer, most of the pinched-off molten
droplet spatters to surroundings, thus causing a remarkable spatter
generation rate. In the globular transfer, a large amount of large-sized
spatters is generated. In the spray transfer, a small amount of spatters
is generated. Accordingly stabilization of spray transfer is a key to
reduce spatter generation rate.

[0005] Independently, the following welding methods have been proposed in
known techniques as methods for performing gas-shielded arc welding at
high current densities typically in fillet welding and multilayer
welding.

[0006] JP-A No. S59 (1984)-45084 proposes a welding method to attain a
high deposition rate (amount of deposited metal) by using a solid wire as
an electrode wire and using a four-component gaseous mixture containing
40 to 70 percent by volume of argon, 25 to 60 percent by volume of
helium, 3 to 10 percent by volume of carbon dioxide, and 0.1 to 1 percent
by volume of oxygen as a shielding gas.

[0007] JP-A No. H03 (1991)-169485 proposes a welding method to attain a
high deposition rate and to attain a bead smoothing effect by the action
of slag, in which welding is performed with a slag-based flux-cored wire
as an electrode wire and carbon dioxide gas as a shielding gas at a
current density of 300 A/mm2 or more.

[0008] JP-A No. H03 (1991)-35881 proposes a welding method to attain a
good penetration shape, in which welding is performed using a solid wire
and a shielding gas at a current density of 300 A/mm2 or more, where
the solid wire has a resistivity ρ of 25 to 65 μΩcm and
contains sulfur (S) in a content of 0.010 to 0.040 percent by mass, and
the sulfur content and the resistivity ρ satisfy the condition: K=20
to 40 wherein K=505S+0.41ρ; and the shielding gas is a gaseous
mixture containing CO2 in a content of 2 to 20 percent by volume and
O2 in a content of 1 to 10 percent by volume with the remainder
being argon (Ar), in which the CO2 content and the O2 content
satisfy the condition: [CO2+2×O2]≧20 percent by
volume.

SUMMARY OF INVENTION

Technical Problem

[0009] The welding method disclosed in JP-A No. S59 (1984)-45084 is
intended to stabilize spray transfer (see FIG. 2C) but fails to improve
or suppress rotating transfer when welding is performed at further higher
current densities, thus causing large amounts of spatters.

[0011] The welding method disclosed in JP-A No. H03 (1991)-169485 is
intended to stabilize the penetration shape through stabilization of
rotating transfer (see FIG. 2A) but fails to suppress generation of small
spatters with rotating transfer, resulting in deposition of large amounts
of spatters in the vicinity of weld beads. Such small spatters, if
deposited, are difficult to remove, leading to poor production
efficiency.

[0012] Accordingly, the present invention has been made to solve these
problems, and an object of the present invention is to provide a method
for high-current-density gas-shielded arc welding, which attains
significant reduction in spatter while providing a high deposition rate.

Solution to Problem

[0013] To achieve the object, the present invention provides, in an
aspect, a high-current-density gas-shielded arc welding method, the
method including the step of performing pulsed arc welding with a
flux-cored wire as an electrode wire, in which the pulsed arc welding is
carried out by passing a pulsed current so that a pulse peak current
density during a pulse peak time Tp is 400 to 950 A/mm2, a pulse
base current density during a pulse base time Tb is 200 A/mm2 or
more and differs from the pulse peak current density by 200 to 400
A/mm2, and an average current density is 350 to 750 A/mm2.

[0014] According to this configuration, the pulse peak current density,
pulse base current density, and average current density in the pulsed arc
welding with the flux-cored wire are set within specific ranges. The
configuration therefore stabilizes the spray transfer to reduce the
spatter generation rate even during welding at high current densities,
and significantly increases the deposition rate as compared to customary
welding methods performed at the same welding current.

[0015] In a preferred embodiment of the high-current-density gas-shielded
arc welding method according to the present invention, the shielding gas
is preferably a gaseous mixture containing CO2 in a content of 5 to
35 percent by volume with the remainder being argon (Ar).

[0016] The configuration employs the specific shielding gas, thereby
reduces the spatter generation rate during pulsed arc welding at high
current densities, and simultaneously suppresses the formation of oxide
and thereby reduces the slag generation rate. In addition, the
configuration improves flexibility and economical efficiency of the
method, since the two-component gaseous mixture of Ar and CO2 is not
a special gas but a widely used gas as the shielding gas.

[0017] In another preferred embodiment of the high-current-density
gas-shielded arc welding method according to the present invention, the
flux-cored wire includes a steel sheath; and a flux filled in the sheath,
and the flux-cored wire has a flux filling rate of 10 to 25 percent by
mass based on the total mass of the wire and contains carbon (C) in a
content of 0.08 percent by mass or less, silicon (Si) in a content of 0.5
to 1.5 percent by mass, manganese (Mn) in a content of 1.5 to 2.5 percent
by mass, and titanium (Ti) in a content of 0.1 to 0.3 percent by mass.

[0018] The flux-cored wire used in this embodiment has the predetermined
chemical composition, thereby helps to reduce turbulence in droplet
transfer during the pulsed arc welding and to reduce the spatter
generation rate, and simultaneously reduces the slag generation rate. In
addition, this configuration gives weld beads having good shapes.

[0019] The high-current-density gas-shielded arc welding method according
to the present invention performs pulsed arc welding at current densities
within the predetermined range, thereby achieves significant reduction in
spatter while providing a high deposition rate. As a result, the method
attains welding with efficiency equal to or higher than that in customary
methods and eliminates the need of extra time and effort to perform a
process of removing spatter, resulting in further improved efficiency in
the welding process. In addition, the method eliminates the need of extra
time and effort to perform a process of removing slag in multilayer
welding and gives weld beads with beautiful appearance.

BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG. 1 is a schematic view showing the designations of a pulse
waveform in the high-current-density gas-shielded arc welding method
according to the present invention;

[0023] Initially, welding equipment for use in the high-current-density
gas-shielded arc welding method according to the present invention will
be described. The welding equipment is not limited, as long as being
welding equipment for performing gas-shielded pulsed arc welding, and may
be known or customary welding equipment.

[0024] As illustrated in FIG. 3, the welding equipment 100 includes, for
example, a welding torch 106, a robot 104, a wire feed unit 101, a
welding power source 102, and a power-source control unit 103. The
welding torch 106 has, both at the tip thereof, a flux-cored wire 108
working as a consumable electrode; and a shielding gas nozzle (not shown)
for feeding a shielding gas, arranged around the outer periphery of the
flux-cored wire 108. The welding torch 106 is attached at the tip of the
robot 104, and the robot 104 moves the welding torch 106 along the weld
line on a workpiece 107. The wire feed unit 101 feeds the flux-cored wire
108 to the welding torch 106. The welding power source 102 feeds a pulsed
current via the wire feed unit 101 to the flux-cored wire 108 to generate
pulsed arc between the flux-cored wire 108 and the workpiece 107. The
power-source control unit 103 controls the pulsed current of the welding
power source 102. The welding equipment 100 may further include a robot
control unit 105 to control the operation of the robot for moving the
welding torch 106. The power-source control unit 103 and the robot
control unit 105 include, for example, a central processing unit (CPU),
read-only memory (ROM), random-access memory (RAM), hard disk drive
(HDD), and input-output interface.

[0025] The high-current-density gas-shielded arc welding method is
featured by controlling the pulsed current in the power source control
unit under predetermined conditions when pulsed arc welding is performed
using the welding equipment. More specifically, the method is featured by
specifying the pulsed current densities within the specific ranges. The
high-current-density gas-shielded arc welding method according to the
present invention will be illustrated below.

[0026] The high-current-density gas-shielded arc welding method performs
pulsed arc welding with a flux-cored wire as an electrode wire, in which
the pulse peak current density during a pulse peak time, the pulse base
current density during a pulse base time, and the average current density
in the pulsed current in the pulsed arc welding are specified or
controlled within the specific ranges.

[0027] As used herein the term "pulse" refers to a current waveform as
illustrated in FIG. 1, which is formed from a pulse power source and
which includes repeating rectangular or trapezoidal forms (rectangular
forms in the example of FIG. 1). Herein, a pulse peak time Tp and a pulse
peak current Ip are defined as the time (duration) and the current during
the top of the rectangular or trapezoidal wave; a pulse base time Tb and
a pulse base current Ib are defined as the time and the current during
the bottom of the wave; and an average current Ia is defined as an
average with time of the time integration of the welding current. The
following equation holds for the rectangular wave in FIG. 1:
Ia=(IpTp+IbTb)/(Tp+Tb). A current density is obtained by dividing each
current value by the sectional area of the current path in the wire.

[0028] Reasons why such a flux-cored wire is used in the present invention
will be described below.

[0029] High-current-density gas-shielded arc welding, if using a solid
wire, presents rotating transfer and thereby causes large amounts of
spatters. The rotating transfer (phenomenon) is caused by the uniform
cross section of the solid wire and by the action of electromagnetic
force of unstable arc. Specifically, the tip of the solid wire having a
uniform cross section is prone to be softened and to elongate at high
current densities, and if arc, which becomes unstable by the action of
the high current, deflects even slightly, the elongated tip molten metal
2 receives centripetal force through an interaction between the formed
magnetic field and the high current passing through the tip molten metal
2. Once the tip molten metal 2 begins to swing even slightly by the above
process, it begins to rotate stationarily by the action of Lorentz force
(see FIG. 2A).

[0030] The rotating phenomenon is prone to occur at an average current
density of 300 A/mm2 or more. When pulsed-current welding is adopted
to this case, the welding neither presents pulsed spray transfer (see
FIG. 2C) as in welding at current densities of less than 300 A/mm2
nor suppresses the rotating phenomenon (see FIG. 2A). The rotating
phenomenon may be contrarily accelerated by the high current during the
pulse peak time Tp (see FIG. 1) in the pulsed-current welding. For these
reasons, the use of the solid wire hardly achieves reduction in spatter
generation even when pulsed arc welding is adopted at high current
densities.

[0031] In contrast, a flux-cored wire includes a cylindrical or tubular
steel sheath; and a flux filled in the cylindrical sheath. The flux-cored
wire hereby has a nonuniform cross section with the flux occupying the
core portion, thereby shows a discontinuous temperature distribution
profile in the wire cross section, and reduces the phenomenon in which
the extending portion is softened and melts to allow the tip of the wire
to elongate, even at high current densities. When pulsed arc welding is
adopted to this technique, the arc can have higher arc stiffness by the
action of the plasma stream and the magnetic field formed by the arc
itself. This inhibits an unstable arc even with high current densities
and thereby does not cause the trigger of the rotating phenomenon. This
allows pulsed spray transfer even at high current densities, and the
resulting molten droplet is smoothly pinched off from the wire tip by the
action of high electromagnetic pinch force during the pulse peak time Tp
and is absorbed by the molten pool. Furthermore, the use of the pulsed
current increases the Joule heating effect in the wire extending portion
and thereby increases the deposition rate when welding is performed at
the same average current. For these reasons, the flux-cored wire is used
in the present invention. As used herein the term "arc stiffness" refers
to such a directivity that arc is generated toward the wire feeding
direction regardless of the inclination of the welding wire with respect
to the base metal (workpiece).

[0032] However, such a flux-cored wire has the flux core which does not
substantially allow a current to pass therethrough, and therefore a high
current passes mainly through the steel sheath. The resulting current
passes at very high current densities, and this may cause an unstable
phenomenon of local melting of the steel sheath alone, to impair the arc
stability and to cause spatter generation. This phenomenon is caused by
excessive Joule heating in the steel sheath and occurs particularly when
such customary pulse peak current Ip and pulse base current Ib as
generally adopted to pulsed arc welding using the solid wire are adopted
without optimization, and whereby there is a large difference in current
density of 400 A/mm2 or more between the pulse peak time Tp and the
pulse base time Tb. Accordingly, the adaptation of customary current
waveforms used in common pulsed arc welding as intact fails to reduce
spatters although it inhibits the rotating phenomenon.

[0033] The present inventors found that gas-shielded arc welding even when
performed at high current densities may allow spray transfer by using a
flux-cored wire to reduce the frequency of elongation of the tip molten
metal and by using a pulsed current to increase the stiffness of arc
itself and to suppress the unstable arc, as described above. However,
when customary pulsed current waveforms as adopted to the customary solid
wires are adopted herein, the unstable phenomenon due to local melting of
the steel sheath occurs, and this impedes the reduction of spatter,
although the rotating phenomenon is prevented. The present inventors made
intensive investigations, focused on the pulse peak current density
during the pulse peak time Tp, the pulse base current density during the
pulse base time Tb, and the average current density, specified optimal
ranges of the respective current densities optimum for the flux-cored
wire, and thereby discovered a welding method by which spatters can be
reduced even in gas-shielded arc welding at high current densities.

[0034] Specifically, optimal ranges are such that the pulse peak current
density during the pulse peak time Tp is 400 to 950 A/mm2; the pulse
base current density during the pulse base time Tb is 200 A/mm2 or
more and differs from the pulse peak current density by 200 to 400
A/mm2; and the average current density is 350 to 750 A/mm2.
Pulsed arc welding performed at current densities within the specific
ranges extremely less causes spatter generation, because the welding
presents stable spray transfer in which the steel sheath melts uniformly
and forms a molten droplet together with the molten flux at the tip of
the wire, and the molten droplet is smoothly pinched off by the action of
the electromagnetic pinch force during the pulse peak time Tp. In
addition, the welding gives an effectively increased deposition rate by
employing the pulse welding technique. Reasons why the ranges of the
current densities are specified will be described below.

[0035] Pulse Peak Current Density: 400 to 950 A/mm2

[0036] A pulse peak current density, if being less than 400 A/mm2,
causes insufficient arc stiffness and fails to give a sufficiently
improved deposition rate due to the pulse welding technique. A pulse peak
current density, if being more than 950 A/mm2, results in an
excessively high current density, thereby causes nonuniform melting of
the steel sheath, impairs the arc stability, and increases the spatter
generation rate.

[0037] Pulse Base Current Density: 200 A/mm2 or more with difference
from pulse peak current density of 200 to 400 A/mm2

[0038] A pulse base current density, if being less than 200 A/mm2,
causes insufficient arc stiffness during the pulse base time Tp, induces
unstable arc, and increases the spatter generation rate. If the
difference from the corresponding pulse peak current density is less than
200 A/mm2, a desired improved deposition rate due to the pulse
welding technique may not be obtained. If the difference from the pulse
peak current density is more than 400 A/mm2, the steel sheath may
melt nonuniformly to increase the spatter generation rate.

[0039] Average Current Density: 350 to 750 A/mm2

[0040] Welding, if performed at an average current density of less than
350 A/mm2, results in an insufficient deposition rate. Welding, if
performed at an average current density of more than 750 A/mm2,
gives an excessively large deposition rate, impedes an effective arc
digging effect, causes weld defects such as incomplete penetration and
lack of fusion in multilayer welding, and increases the spatter
generation rate.

[0041] The shielding gas for use in the present invention is not limited
typically on its type and chemical composition. In a preferred
embodiment, the shielding gas is a gaseous mixture containing CO2 in
a content of 5 to 35 percent by volume with the remainder being Ar. The
use of the gaseous mixture having such a chemical composition further
reduces the spatter generation rate and slag generation rate in the
pulsed arc welding.

[0042] Shielding Gas: CO2 in a Content of 5 to 35 Percent by Volume
with the Remainder being Ar

[0043] If the shielding gas has a CO2 content of less than 5 percent
by volume, the arc may be prone to creep upward the molten droplet, and
this may cause the wire tip to melt and be softened to thereby elongate,
thus causing rotating transfer even when a flux-cored wire is used. This
may often cause the arc to be unstable to cause large amounts of spatters
and may cause the arc to meander, resulting in nonuniform shapes of
beads. CO2, if contained in a content of more than 35 percent by
volume, acts as an oxidizing gas and may cause an endothermic reaction
due to its molecular dissociation. The endothermic reaction may cool the
arc, thereby often cause the transfer mode of the molten droplet to be
globular transfer (see FIG. 2B), and often cause large spatters. In
addition, CO2 serving as an oxidizing gas, if contained in a such a
high content, may often form oxides and often cause a large amount of
slag.

[0044] The flux-cored wire for use in the present invention is not limited
in its conditions or parameters such as chemical composition, material of
the steel sheath, ratio of the cross-sectional area of the steel sheath
to the total cross-sectional area of the wire, wire cross-sectional
shape, wire diameter, and filling rate of the flux.

[0045] In a preferred embodiment, the flux-cored wire has a flux filling
rate of 10 to 25 percent by mass based on the total mass of the wire and
contains C in a content of 0.08 percent by mass or less, Si in a content
of 0.5 to 1.5 percent by mass, Mn in a content of 1.5 to 2.5 percent by
mass, and Ti in a content of 0.1 to 0.3 percent by mass. The flux-cored
wire in this embodiment contains C, Si, Mn, and Ti as above, with the
remainder being iron (Fe) and inevitable impurities. The flux-cored wire,
as having the above-specified chemical composition, helps to reduce the
spatter generation rate and slag generation rate and to give weld beads
with good shape. The above elements, i.e., C, Si, Mn, Ti, and Fe are
contained in at least one of the steel sheath and the flux.

[0046] Flux Filling Rate: 10 to 25 Percent by Mass

[0047] The flux-cored wire, if having a flux filling rate of less than 10
percent by mass, may impair the arc stability to increase the spatter
generation rate and may often cause poor appearance of beads. The
flux-cored wire, if having a flux filling rate of more than 25 percent by
mass, may tend to be broken.

[0048] Carbon (C) Content: 0.08 Percent by Mass or Less

[0049] Carbon (C) element is contained in or as, for example, steel
sheath, ferromanganese, ferrosilicomanganese, and iron powder and is
important to ensure the strength of the weld metal. Particularly in the
high-current-density gas-shielded arc welding with an Ar--CO2
gaseous mixture, carbon significantly affects the arc stability and is
thereby necessary for ensuring arc concentration and arc stability.
However, carbon, if present in a content of more than 0.08 percent by
mass, may be apt to react with oxygen in the shielding gas to form
gaseous carbon monoxide (CO), and the carbon monoxide may often be
released from the molten droplet to disturb the droplet transfer, often
resulting in an increased spatter generation rate. The carbon content is
more preferably 0.02 percent by mass or more for further better arc
stability.

[0050] Silicon (Si) Content: 0.5 to 1.5 Percent by Mass

[0051] Silicon (Si) element is fed typically from or as the steel sheath,
metal silicon, ferrosilicon, and ferrosilicomanganese. This element is
necessary for ensuring the strength of the weld metal and is also
necessary as a deoxidizer. This element also acts for improving the
wettability of beads. The flux-cored wire, if having a Si content of less
than 0.5 percent by mass, may cause insufficient strength of the weld
metal, and, in gas-shielded arc welding at high current densities of 350
A/mm2 or more, may cause insufficient deoxidization to thereby
invite defects such as blowholes. In addition, this flux-cored wire may
cause the molten droplet to be pinched off unsmoothly and to elongate at
its tip, thus increasing the spatter generation rate slightly. This
flux-cored wire may also cause poor wettability of beads and may often
fail to give beads with beautiful shape. In contrast, the flux-cored
wire, if having a Si content of more than 1.5 percent by mass, may often
increase the slag generation rate.

[0052] Manganese (Mn) Content: 1.5 to 2.5 Percent by Mass

[0053] Manganese (Mn) element is fed typically from or as the steel
sheath, metal manganese, ferromanganese, and ferrosilicomanganese. This
element is necessary for ensuring the strength and toughness of the weld
metal and is also necessary as a deoxidizer. The flux-cored wire, if
having a Mn content of less than 1.5 percent by mass and being used in
gas-shielded arc welding at high current densities of 350 A/mm2 or
more, may cause insufficient deoxidization and may thereby cause defects
such as blowholes. In this case, the molten droplet may be pinched off
unsmoothly and be apt to elongate at its tip, thus resulting in a
slightly increased spatter generation rate. In contrast, the flux-cored
wire, if having a Mn content of more than 2.5 percent by mass, may often
increase the slag generation rate.

[0054] Titanium (Ti) Content: 0.1 to 0.3 Percent by Mass

[0055] Titanium (Ti) element is fed typically from or as the steel sheath,
metal titanium, ferrotitanium, and TiO2. This element serves as a
strong deoxidizer and is necessary for ensuring the strength and
toughness of the weld metal. Specifically, the flux-cored wire, if having
a Ti content of less than 0.1 percent by mass, may cause the molten
droplet to be pinched off unsmoothly and to elongate at its tip, thus
resulting in a slightly increased spatter generation rate. In contrast,
the flux-cored wire, if having a Ti content of more than 0.3 percent by
mass, may increase the slag generation rate and may often impair the slag
removability. The Ti content is indicated in terms of the content of
metal titanium.

[0056] The flux-cored wire may further contain, in addition to the above
chemical composition, any of components to be contained in regular
flux-cored wires, such as slag-forming materials, deoxidizers, and
fluorides. The flux-cored wire preferably has a wire diameter of 1.2 to
1.6 mm, for a high deposition rate at high current densities.

[0057] The lengths (durations) of the pulse peak time Tp and pulse base
time Tb do not significantly affect the advantageous effects of the
present invention and are not critical. However, the pulse peak time Tp
is preferably set to be 0.5 to 10 ms, because the long-term application
of the pulse peak current Ip may cause the arc length to vibrate. The
pulse base time Tb may be set in accordance with the set pulse peak time
Tp, so as to give desired average welding current and voltage.

Experimental Example 1

[0058] A first experimental example according to the present invention
will be described below.

[0059] An arc welding was performed under welding conditions below using
the shielding gas and pulse parameters (pulse current densities) given in
Table 1, and the spatter generation rate and deposition rate were
measured and evaluated. The results are shown in Table 1.

[0060] The spatter generation rate is determined by performing
bead-on-plate welding in a copper box and measuring the weight of
spatters collected in the copper box. A sample having a weight of
collected spatters of 1 gram per minute or more was evaluated as having a
high spatter generation rate (poor suppression in spatter generation:
Poor); and one having a weight of collected spatters of less than 1 gram
per minute was evaluated as having a low spatter generation rate (good
suppression in spatter generation: Good).

[0061] The deposition rate was evaluated based on the change in weight of
the specimen between before and after welding. A sample having a weight
change of less than 150 grams per minute was evaluated as having a low
deposition rate (Poor); and one having a weight change of 150 grams per
minute or more was evaluated as having a high deposition rate (Good).

[0062] As an assessment, a sample evaluated as good both in spatter
generation rate and in deposition rate was evaluated as being accepted
(Acpt); and one evaluated as poor in at least one of spatter generation
rate and deposition rate was evaluated as being rejected (Rej).

[0063] Welding Conditions:

Wire (solid): JIS Z3312:1999 YGW11 Wire (FCW): JIS Z3313:1999 YFW-C50DM,
having a carbon content of 0.04 percent by mass, a Si content of 1.0
percent by mass, a Mn content of 2.0 percent by mass, and a Ti content of
0.2 percent by mass, and having a flux filling rate of 15 percent by mass
Specimen (base metal; workpiece): SS400 25-mm thick

[0065] As is demonstrated from Table 1, Examples of Sample Nos. 17 to 23
were prepared under conditions satisfying the requirements in the present
invention, thereby had low spatter generation rates and high deposition
rates, and were accepted.

[0066] In contrast, Comparative Examples of Sample Nos. 1 to 16 prepared
under conditions not satisfying the requirements in the present invention
were rejected as mentioned below. Specifically, Comparative Examples of
Sample Nos. 1 to 3 were prepared through welding with an Ar--CO2 gas
mixture as a shielding gas and with a solid wire, whereby showed high
spatter generation rates due to rotating transfer, and were rejected.
Comparative Examples of Sample Nos. 4 and 5 were prepared while using 100
percent by volume CO2 as a shielding gas, thereby showed high
spatter generation rates due to globular transfer, and were rejected.
Comparative Examples of Sample Nos. 6 and 7 were prepared without using a
pulsed current as the welding current, thereby showed high spatter
generation rates, and were rejected.

[0067] Comparative Examples of Sample Nos. 8 and 9 were prepared at a
pulse base current density of less than the lower limit and with a
difference between the pulse peak current density and the pulse base
current density of more than the upper limit, thereby showed high spatter
generation rates due to nonuniform melting of the steel sheath, and were
rejected. Comparative Example of Sample No. 10 was prepared at an average
current density of less than the lower limit, thereby showed a low
deposition rate, and was rejected. Comparative Example of Sample No. 11
was prepared at a pulse peak current density, a pulse base current
density, and an average current density of respectively less than the
lower limits, showed a low deposition rate, had a high spatter generation
rate, and was rejected. Comparative Example of Sample No. 12 was prepared
at a pulse base current density of less than the lower limit, thereby had
a high spatter generation rate, and was rejected. Comparative Example of
Sample No. 13 was prepared at a pulse peak current density, a difference
between the pulse peak current density and the pulse base current
density, and an average current density of respectively less than the
lower limits, thereby showed a low deposition rate, and was rejected.
Comparative Example of Sample No. 14 was prepared at a pulse peak current
density and a difference between the pulse peak current density and the
pulse base current density of respectively less than the upper limits,
thereby showed a high spatter generation rate, and was rejected.
Comparative Example of Sample No. 15 was prepared at a pulse peak current
density, a pulse base current density, and an average current density of
respectively less than the lower limits, thereby showed a high spatter
generation rate and a low deposition rate, and was rejected. Comparative
Example of Sample No. 16 was prepared at an average current density of
more than the upper limit, thereby had a high spatter generation rate,
and was rejected.

Experimental Example 2

[0068] A second experimental example according to the present invention
will be described below.

[0069] An arc welding was performed under welding conditions below using
the flux-cored wires given in Table 2, and the spatter generation rate
and slag generation rate were measured and evaluated by the procedure of
Experimental Example 1. The results are shown in Table 2.

[0070] The spatter generation rate was measured and determined by the
procedure of Experimental Example 1. A sample having a spatter generation
rate of 0.6 gram per minute or more and less than 1 gram per minute was
evaluated as having a low spatter generation rate (good suppression in
spatter generation; Good); and one having a spatter generation rate of
less than 0.6 gram per minute was evaluated as having a further low
spatter generation rate (excellent suppression in spatter generation;
Excellent).

[0071] The slag generation rate was determined by performing two-layer
two-pass welding (welding length of 30 cm) in a single-bevel groove with
a groove angle of 35 degrees, collecting the whole quantity of generated
slag, and measuring the weight of the collected slag. A sample having a
slag weight of 7 grams or less was evaluated as having a low slag
generation rate (excellent in suppression of slag generation; Excellent);
and one having a slag weight of more than 7 grams was evaluated as having
a somewhat high slag generation rate (good in suppression of slag
generation; Good).

[0072] As the assessment, a sample evaluated as excellent both in spatter
generation rate and slag generation rate was evaluated as being excellent
(Excellent); and one evaluated as good in at least one of spatter
generation rate and slag generation rate was evaluated as being good
(Good).

[0077] As is demonstrated from Table 2, of Examples of Sample Nos. 24 to
38 prepared under conditions satisfying the requirements in the present
invention, Examples of Sample Nos. 31 to 38 employed the flux-cored wires
having chemical compositions within the preferred range in the present
invention had lower spatter generation rates and lower slag generation
rates and were assessed as excellent, as compared to Examples of Sample
Nos. 24 to 30 employing the flux-cored wires having chemical compositions
out of the preferred range. As measured by the procedure of Experimental
Example 1, Examples of Sample Nos. 24 to 38 all had high deposition rates
of 150 gram per minute or more.